Elves, fairies, gremlins, house wights...
Lots of cultures have them, and all it costs me to keep them happy is a bowl of milk and honey set out every so often. Superstitious? You betcha. But, most folks have an invisible friend in the sky or two already, so, in for a penny, in for a pound, I say...
Of course, it could be you've just been lucky until now...